An electronic device, such as a computer system, may experience serious operating difficulties in the presence of unintended radio-frequency fields. Such fields, which interfere with the normal operation of the device, are generally known as electromagnetic interference ("EMI"). Numerous techniques, which generally increase the manufacturing cost of the device, are used to ensure that normal operation of the electronic device is not compromised.
For example, some immunity to EMI may be provided by mounting the electronic devices inside a shielded enclosure known as a Faraday cage. Such an enclosure is usually constructed with special conductive gaskets which electrically join the access panels of the enclosure which are particularly disposed to unintended electromagnetic leakage. In addition, the electrical cables of the device, which join the device with other equipment and power sources, must be designed with specialized EMI filters. EMI shields and filters may provide some immunity to unintended electromagnetic noise, however, it is not always technically possible or economically practical to design a device with total immunity to EMI.
Therefore, it is necessary to measure to what extent the operation of an electronic device is effected by residual EMI. In order to determine the susceptibility of the device to EMI, the device must be subjected to field strengths which are representative of the ambient environments in which it will be operated. The test fields to which the device is subjected should be predictable so that the level of radiation is determinable to a high degree of certainty.
EMI susceptibility testing is typically performed inside an enclosed space or "screen room" which has good electrical isolation from external radiation. Notwithstanding the electrical isolation of the screen room, serious measurement problems and errors can still result due to the electrical conductivity and reflectivity of the walls of the room which set up standing waves which may interfere with the fields being measured.
In order to minimize these measurement problems, anechoic screen rooms for measurements at frequencies of a few megahertz have been constructed underground, in caves or in tunnels. The nonmetallic, lossy and irregular walls of the room reduce reflection and attenuate fields which may interfere with the testing measures. However, access to naturally occurring screen rooms may not always be readily available and geological considerations may otherwise decide against the economical construction of such an underground screen room.
In another technique, the device to be tested is placed inside a transverse electromagnetic transmission (TEM) cell which has a known uniform transverse electromagnetic field. However, since the device to be tested must be placed entirely within the cell, the size of the device that can be tested in a TEM is considerably reduced as a function of the cell size.
Alternatively, susceptibility testing is performed in an anechoic chamber. An anechoic chamber is typically a screen room constructed with walls that absorb or attenuate reflected electromagnetic radiation. In one such construction, the walls are covered with a material which has an impedance of about 377 ohms per meter, the characteristic impedance of free space and spaced at one-quarter wavelength from a reflecting wall, a so called quarter wave or Salisbury screen. In such a screen room electromagnetic reflection is prevented by a mechanism of destructive interference. Due to the need to space the screen by one quarter wavelength from the wall, this technique only provides the desired attenuation within a narrow frequency band. More complex structures can be created, at a far greater cost, by placing a plurality of sheets, having varying direct current resistivities, in the range of about 40 to 2,000 ohms, at different distances from the reflecting surfaces. Broadband electromagnetic attenuating materials, which are sometimes used in stealth technologies, are effective for a wider range of electromagnetic frequencies, however such materials are relatively expensive and not always practical for industrial applications.
An anechoic screen room can also be constructed by placing numerous three dimensional absorbing structures inside the room. Typically, the absorbers, each of which has the shape of a triangular pyramid or serrated cone (deformed triangular pyramid) are attached to the walls, ceiling and floor of the room. To effectively absorb electromagnetic radiation in the range of about 30 to 1000 megahertz (a wave length range of about 10 meters to 0.3 meters) the cones need to have sizes set to a value from approximately one-quarter of the maximum wavelength. In other words, cones for absorbing waves in the radio-frequency spectrum can be as large as 2.5 meters. Rooms constructed in this manner for susceptibility testing of large electronic devices, such as complete computer systems, tend to be voluminous and expensive.
Yet another technique requires that a screen room be constructed with ellipsoidal (egg-shaped) reflecting walls. The inner surfaces of the metallic walls define a closed space with two focal points. A source of radio-frequency radiation is placed at a first focal point and a spherical (ball) shaped absorbing structure is disposed at a second focal point. The device to be tested is placed between the two focal points to receive direct radiation from the source at the first focal point. Any waves reflected at the inner surfaces of the walls are directed primarily at the absorbing second focal point thus reducing secondary reflections and resonance within the testing room.
Although some of these techniques partially address the control of the electromagnetic environment within the screen room, the construction of rooms using these and other conventional techniques is generally expensive. In addition, these techniques do not readily permit the simultaneous and rapid adjustment of the phase and amplitude of the radiation to create a variety of field impedances for wide spectrum EMI susceptibility testing. In addition, anechoic rooms with a single point source radiating element generally require high-power amplifiers and its known that point sources create non-linear (inconsistent amplitude over varying distances) fields, which may not be representative of actual ambient operating conditions being modeled.
Therefore, it is desirable to provide an apparatus for susceptibility testing of an electronic device which generates uniform and predictable radio-frequency fields at a relatively low cost. It is also desirable that these fields be adjustable for phase and amplitude distortions over a wide range of field impedances. It also desirable that high flux fields be obtainable without the use of high power generators and amplifiers.